Introduction

This is the second and final report of geophysical fieldwork carried out at Chichen Itza from February 4-10, 1993 by L. Desmond, W. Sauck, J. Callaghan, J. Muehlhausen and K. Zschomler as part of the Instituto Nacional de Antropologia e Historia Proyecto Chichen Itza.  Report I included the results of resistivity testing of areas near the northeast corner of the Castillo Pyramid and the center of the Great Ball Court, and of a ground penetrating radar survey of the Great Plaza in areas: east-west transects 39N to 69N (north of the Castillo Pyramid), and from north-south transect 70E (Temple of the Warriors) to north-south transect 70W, and an area west and north of the northwest corner of the Platform of Venus (Report I, Figures 4, 5 and 6).

This report presents results of the ground penetrating radar survey in the following areas (Figure 1):
1. Castillo Pyramid vicinity.  Area from transect 00N to 39N, west from 00W to north-south transect 70W, and east from 00E to 75E.
2. Castillo Pyramid vicinity.  North-south transect 70W from 00N to 65S.
3. Castillo Pyramid vicinity.  Two transects from 00W to 100W, 65S and 70S (south side of Castillo).
4. Great Ball Court central area.  North-south transect set 10 meters east of the west wall and north-south transect set 10 meters west of the east wall.  Two transects east-west in the center of the court; one transect between the rings set into the east and west walls and the other 10 meters south and parallel to that line.
5. Great Ball Court end zones.  One transect in each end zone and parallel to the east-west wall in each area.  The north end line was 15.4 meters south of the north wall, and the south end line was 18 meters north of the south wall.

Ground Penetrating Radar Survey Results

1.  North from the Castillo Pyramid datum 00N to 39N, west from 00W to north-south line 70W, and east from 00E to 80E (Figure 2).

Contour lines around the Castillo Pyramid show some areas of the limestone bedrock surface just east of the pyramid and near the west side of the Temple of the Warriors Colonnade, and more of the high area noted in Report I to the southwest of the Platform of Venus (Report I, Figure 4, Photo 1).  A mound in the bedrock can be seen about 7 meters north of the pyramid on line 15W, and a trough runs roughly northeast about 20 meters north of the pyramid between 10W and 30W.  While we do not have contour information on the bedrock under the Temple of the Warriors or Colonnade the bedrock becomes shallower as one nears those structures and it seems probable that they are built on high areas of the original limestone topography (Photo 2).

A series of seven transects which were run from west to east from the Castillo Pyramid line 00E crossed an anomalous feature centered about 20 meters east of the Castillo with a north-south extension of about 30 meters (Figure 2).  All seven transects show a disruption of the paleosoil reflector for widths varying from about 5 to 10 meters (Photos 3, 4, and 5).  In all cases, the area below the disturbed zone shows inwardly convergent dipping reflectors, typical of a collapsed cultural feature or trench (Figures 3 and 4).  The reflectors above the disturbed bedrock zone show undisturbed fill which indicates the feature was made prior to construction of the Great Plaza.

The prominent feature indicated by fairly strong reflections at the presumed bedrock surface may actually be the dark soil layer described in Earl Morris' report The Temple of the Warriors (Morris 1931:167).  Contact between limestone rubble used to build the Great Plaza and the massive limestone of the bedrock would not provide a large contrast in electrical properties, and therefore, the strong echo we see would not be present if soil were not present.  Also, the strong low-frequency reflection earlier referred to as "bedrock" is similar to water table signatures observed in other projects using this radar system.  However, this cannot be a "perched" water table because of the undulating nature of the reflector, and because the high hydraulic conductivity of the underlying limestone would not allow water to accumulate here.  The observation by Morris of the 55 cm of black soil in a test pit northeast of the Castillo Pyramid invites a re-interpretation of the nature of this low-frequency reflection.  The reported dark soil layer which we assume is clay-bearing paleosoil could retain sufficient moisture to produce a strong reflection which mimics that due to a sudden relative permittivity increase at a water table boundary.  A variable thickness paleosoil would also facilitate the explanation of the variable amplitude of this reflector across the site.  It commonly weakens across the bedrock knobs and is most intense at the lower levels where the "ponded" residual soils are apt to be thicker and more moist.

Regarding the anomalous feature 20 meters east of the northeast corner of the Castillo Pyramid, the disruption of the reflector might be due to the natural process of downward loss of paleosoil into a fissure in the limestone, or else it implies removal of that soil horizon by the Maya prior to the earthmoving stage of Plaza construction.  The latter may well be the case since four of the lines crossing the feature (10S, 05S, 00N, and 05N) show a change from the normal low-frequency doublet pulse to at least a triple, thicker low-frequency reflector just to the east of the anomalous feature which we hypothesize is where the excavated soil was thrown (Figures 3 and 4).

2. Castillo Pyramid vicinity.  North-south transect 70W from 00N to 65S (Figures 1 and 2).

The single transect indicates a rise in the bedrock near the west stairway at 30S, and deeper fill between 40S and 60S.

3. Castillo Pyramid vicinity.  Two east-west transects from 00W to 100W, 65S and 70S (south of Castillo) (Figure 1).

Reflections indicate almost no paleosoil unlike what we find to the north and east of the Castillo, and plaza fill is one to three meters in thickness in area the surveyed.  Between 75W and 85W is a steep easterly inclining reflector which ranges in depth
from approximately three meters at 85W to six meters at 75W (Figure 5).

4. Great Ball Court central area.  North-south transect set 10 meters east of the west wall and north-south transect set 10 meters west of the east wall.  Two transects east-west in the center of the court.  One transect between the rings set into the east and west walls and the other 10 meters south and parallel to that line (Figure 1).

These transects show bedrock highs or knolls less than a meter from the surface at 74 meters at midcourt (Figure 6), and also at 108 meters (Figure 7).  Between these highs the prominent reflection is at about 3.5 meters depth.

5. Great Ball Court end zones.  One transect in each end zone and parallel to the east-west wall in each area (Figure 1).  The north end zone line was 15.4 meters south of the north wall, and the south end line was 18 meters north of the south wall.

The east-west transect in the north end zone indicates a depression, and loss of amplitude of the paleosoil reflections about 10 meters west of the east wall (Figure 8).  Below this zone are steeply dipping, centrally convergent reflections very similar to ground penetrating radar patterns published by Benson and Yuhr (Benson and Yuhr 1992:478-79) over a subsidence zone developed over vertical "piping" in limestone.  They interpret these patterns as near surface indicators of active cavern formation at greater depth.  Another indication of cavity formation may be the downward loss of fine paleosoil and water into the cavity in this area indicated by the loss of amplitude of the paleosoil reflections.

Conclusions

Ground penetrating radar was very successful in showing considerable internal structure within the fill of the Great Plaza.  It also mapped the approximate depth to bedrock (or to paleosoil on bedrock), revealing more than 4 meters of pre-Plaza topographic relief.  A number of discrete radar features such as "blocks" set above the bedrock reflector, and depressions into that reflective surface merit excavation.  A much larger feature 20 meters east of the northeast corner of the Castillo may well be a fissure or tunnel system, as the bedrock reflector (or paleosoil) is breached along a 30 meter north-south axis as indicated by the disrupted reflector which shows inwardly dipping collapse-type features.

Radar penetration was not sufficient to reach the water table and hence the level of active cavern formation.  Buried electrical lines and pipes in the Plaza caused only minor local disruption or "ringing" of the antenna and are readily recognized.
 Future work should employ a bistatic antenna system to avoid loss of the shallow reflections (first 25 nanoseconds of return when using a monostatic antenna).  We did not ship a second 100 MHz antenna to Chichen Itza to make use of the advantages of a bistatic array because the size would have complicated our travel arrangements.  The Geophysical Survey Systems, Inc. (GSSI) 300 MHz antennas would probably be ideal for this site, and they could be transported in a smaller container.

Our survey indicates an enormous amount of earth was transported by the Maya to build the Great Plaza.  The survey covered approximately 250 meters north/south by 350 meters east/west or about 8.75 hectares of the Great Plaza.  Based on an average fill thickness of 2.5 meters, Sauck estimates that 220,000 cubic meters of soil and rubble weighing about 440,000 metric tons was deposited in the area in order to cover the undulating limestone bedrock and create a flat plaza.  The final depth of the plaza is the result of a number of layers or floors added by the Maya over a considerable amount of time, and is part of the overall plan devised by the Maya to provide open space for religious and political assemblies, and to create a setting which would enhance their city with its temples and pyramids.

Too fully comprehend the extent of the planning and labor involved in the creation of such a magnificent center the full extent of the Great Plaza should be surveyed using geophysical techniques.  Ground penetrating radar has proven to be the method of choice at Chichen Itza because of its ability to readily detect buried parts of structures, foundations, sacbes, large cut stones, natural or human made underground chambers, thickness of fill, and to produce a contour map of the bedrock.

At Chichen Itza it is very difficult to determine bedrock or fill by simple observation since much of the plaza is well compacted or covered with grass.  Ground penetrating radar immediately indicates fill and its depth, and can eliminate the need for test pits and other intrusive methods to locate the limits of the plaza.  It can also provide a complete three dimensional model of the entire plaza and not be restricted to selected areas by sampling techniques.  Using improved antennas we may also be able to detect separate floors built during stages in the development of the plaza to help understand Maya construction methods and develop a construction chronology.

 Finally, our current data does not resolve questions about the exact nature of the linear feature in the bedrock east of the Castillo Pyramid so we would recommend a radar survey of that anomaly using 300 MHz antennas at 2 meter transect intervals to generate very detailed data.  On-site analysis using real time and recorded data from such a radar survey can provide pictorial and locational information to guide archaeologists in deciding whether to excavate, and assist in selecting the optimum area of the feature for excavation.

Acknowledgements
See Report I.

References Cited

Benson, Richard C. and Lynn Yuhr
    1992 A summary of methods for locating and mapping fractures and cavities with emphasis on geophysical methods.  In, Ronald S. Bell, editor, Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems, Oakbrook, Illinois, April 26-29, 1992, pp.471-486. Golden: Society of Engineering and Mineral Exploration Geophysicists.

Morris, Earl H., Jean Charlot and Ann Axtell Morris
    1931 The Temple of the Warriors. Publication 406. Washington: Carnegie Institution.